Semiconductor power switching devices, such as Insulated Gate Bipolar Transistors (IGBTs) or Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFETs) are well-known in the art. For example, IGBTs have been the main power semiconductors used in the inverter sections of variable speed AC motor drives and other similar applications. The latest generation of IGBTs includes Trench Gate Field Stop IGBT devices (TG-IGBTs), which are also sometimes referred to as third generation IGBT devices.
The trench gate IGBT devices offer substantial advantages over prior IGBT devices. For example, the trench gate IGBT tends to have a lower on-state voltage requirement. Further, the trench gate IGBT is typically capable of faster on/off switching than other semiconductor devices, including prior generations of IGBT devices. However, the very fast turn-off behavior of the trench gate IGBT device can make maintaining the voltage across the IGBT within the Reverse Bias Safe Operating Area (RBSOA) very difficult. Additionally, the fast turnoff behavior of the trench gate IGBT device can cause parasitic oscillations within connected circuits. Such parasitic oscillations can interfere with an/or cause failure of the gate drive and other control circuits. Moreover, when a free wheel diode is used, as may be common in the inverter section of an AC motor controller, the very fast turn-on behavior of the trench gate IGBT can cause problems with the reverse recovery. For example, the reverse recovery during “turn on” can be very “snappy” because the current during reverse recovery terminates with a high rate of change. This can also cause parasitic oscillations and potential failure of the trench gate IGBT device and gate drive circuit. Such problems are more significant at higher operating voltages and currents.
A number of techniques have been proposed in the art to address some of the issues related to the fast turn-on and turn-off behavior of the trench gate IGBT when used in an inverter. In one technique, the gate resistance of the trench gate IGBT is increased so that the device switches more slowly. Increasing the gate resistance helps to control the turn-on behavior of the IGBT. However, to effect control the turn-off behavior of the trench gate IGBT, the gate resistance has to be substantially increased by as much as 10 to 20 times. This substantial increase in resistance can create delays in the “turn off” of the trench gate IGBT device that may be generally unacceptable.
In another technique, a two-stage “turn on” and “turn off” process can be used to control the switching of the trench gate IGBT devices. In this technique, the value of gate resistor is increased at fixed stages to control the “turn on” or “turn off” of the trench gate IGBT devices. This technique addresses the issue of the unacceptable delay during “turn off” that occurs when only a simple, fixed resistance is used. In yet another technique, the collector voltages of trench gate IGBT device (typically both the absolute value and rate of change of the collector voltage) can be monitored, and the gate voltage is changed to affect turn on/turn off times. In yet another technique, the rate of change of the current in the trench gate IGBT device can be monitored using voltages between the power and the control terminals of the module having the IGBT devices, and the gate voltages can be changed to acceptable levels.
The techniques described above were developed to avoid over-voltage and oscillations in the power circuit under “worst-case” conditions. However, even though a gate drive is designed to survive such worst-case conditions, the power circuit rarely, if ever, experiences such worst case conditions. The vast majority of operating conditions are less (better) than worst case. Thus, the power circuit does not operate optimally when designed for the worst-case condition it will rarely, if ever, experience. Namely, the turn-on and turn-off behaviors of the trench gate IGBT devices are considerably slower than they need to be under operating conditions outside the worst-case conditions. The slow switching behaviors result in increased heat dissipation along with resulting loss of equipment rating and/or reliability.